1. Field of the Invention
This invention relates to microcontrollers, and more particularly to microcontrollers employed within network interface units of devices coupled to computer networks. A microcontroller is presented having blocks of programmable logic that are variably configurable to handle different communication protocols.
2. Description of the Relevant Art
A computer network typically includes two or more computer systems interconnected for the purpose of exchanging data or sharing resources such as input/output (I/O) devices. There are two basic types of computer networks: local area networks (LANs) and wide area networks (WANs). A LAN is an interconnection of computer systems within a limited geographical area. A LAN may encompass a portion of a building, an entire building, or a cluster of buildings. A LAN allows users within the same office or company to exchange data files or share resources such as printers. A WAN connects computer systems over a large area, such as an entire city, state, or country. WANs are typically implemented using existing public switched telephone networks (PSTNs), and are thus operated and maintained by telephone service providers such as AT&T, MCI, and Sprint. A WAN allows users at remote sites, such as retail outlets, branch offices, or even their own homes, to exchange information with other users at a central corporate office or data site. The Internet, a complex interconnection of WANs, allows users to access information provided by companies and other users located worldwide.
Several different types of computer systems exist today, each manipulating and storing data in a different way. The interconnection of such diverse computer systems presents a challenging task. Networking capability is provided by hardware and/or software which implements a standardized interface specifically designed for the exchange of information among heterogeneous computer systems.
In an effort to allow different types of computer systems to communicate with one another, the international standards organization (ISO) developed an open systems interconnection (OSI) reference model. The OSI reference model defines various layers ranging from a lowermost physical layer (network cables and host adapter hardware) to an uppermost application layer (communication access software). Between the physical layer and application layer are various other layers used to implement a standard communication "protocol" used to transfer information between computer systems.
The OSI reference model provides for packet data transmission. In packet-switched networks, information is transferred between computer systems in small portions or "packets". The packets are transmitted sequentially over a transmission medium (e.g., coaxial cable, twisted-pair cable, or fiber-optic cable) interconnecting the network computer systems. Packet switching provides a way for all of the network computer systems to share the full data transmission capability of the transmission medium without interference or interminable waiting periods for access. As a result, packet-switched networks are common today. The OSI model includes a data link layer above the physical layer, and a network layer above the data link layer. The data link layer and the network layer, in combination, define the basic protocol used in the computer network. The network layer divides a block of data to be transmitted into packets. The data link layer is responsible for establishing a link between a transmitting computer system and a receiving computer system, encapsulating the packet data into "frames" which also include error control and flow control signals, transmitting the frames upon the network transmission medium, and deactivating the link when the data transfer is complete. Error control allows for detection of transmission errors. Flow control allows for transmission of data at a rate at which the receiving computer system can handle the incoming data.
A network interface unit within a computer system or other device performs the data-link-layer functions. In a transmit mode, the network interface unit of a transmitting device establishes a link with a network interface unit of a receiving device, encapsulates the network-layer packet data into frames, transmits the frames upon the network transmission medium, and deactivates the link when the data transfer is complete. In a receive mode, the network interface unit receives frames from the network transmission medium, checks the received frame data for error, and reassembles the packets from the frame data. All of the above actions are performed according to a selected communication protocol.
There are numerous communication protocols currently being used in LANs and WANs to effectuate communication among computer systems. Such protocols include TCP/IP, Ethernet, frame relay, DDS, ATM, X.25 and Sonet. Each communication protocol achieves data transmission in a different way, and network hardware and/or software designed to implement one communication protocol will typically not recognize data transmitted using another protocol. In addition, new communication protocols are continuously being developed in an effort to increase the rates and reliabilities of data exchanges.
A microcontroller is an integrated circuit which incorporates a processor core along with one or more support circuits on the same monolithic semiconductor substrate (i.e., chip). The support circuits perform support functions such as communication functions and memory interface functions. Systems which employ microcontrollers are typically implemented using fewer separate semiconductor devices. Advantages of such systems include lower fabrication costs and higher reliabilities. Microcontrollers find applications in industrial and commercial products including control systems, computer terminals, hand-held communications devices (e.g., cellular telephones), network interface equipment, photocopier machines, facsimile machines, and hard disk drives.
Many different types of programmable logic devices (PLDs) exist today. PLDs are general-purpose digital components which are manufactured in an "unprogrammed" state, and are later "programmed" to implement a desired logical function. A PLD is typically programmed by a sequence of electrical pulses which configure one or more arrays of programmable switching elements within the PLD. Examples of different types of PLDs include programmable logic arrays (PLAs), programmable array logic devices (PALs), and field programmable gate arrays (FPGAs). PLAs and PALs have AND and OR arrays. The AND array performs logical AND operations upon input values, forming product terms. The OR array performs logical OR operations upon the product terms, forming output values. The AND and OR arrays of PLAs include programmable switching elements, while only the AND arrays of PALs are programmable. PLAs and PALs implement combinational logic functions as a sum of the product terms (i.e., a sum of products) of input values. FPGAs are semi-custom logic devices including "islands" of programmable logic blocks called "logic cells" surrounded by an interconnection network which includes programmable switching elements. The logical functions performed by the logic cells are determined by programming, as are the interconnections formed between the logic cells. PLAs, PALs, and FPGAs may also include storage elements called flip-flops needed to implement sequential logic functions.
It would be beneficial to have a network interface unit which includes a microcontroller having programmable logic circuitry configurable to implement one of many possible communication protocols. For example, the desired network interface unit could be configured to assemble, transmit, and receive data units (i.e., frames) of one protocol, then later reconfigured to assemble, transmit, and receive data units of another protocol. Reconfiguration is a problem which must be solved in order to allow the network interface unit to be reconfigured to implement different communication protocols. Essentially, the desired reconfigurable network interface unit must be included within network cards of computer systems of a LAN as well as any router which links the LAN to another computer system via, for example, the public switched telephone network (PSTN).